The present invention relates to electrical products, and more particularly, to electrical connectors for electrical systems and associated methods.
An electrical distribution system typically includes distribution lines or feeders that extend out from a substation transformer. The substation transformer is typically connected to a generator via electrical transmission lines.
Along the path of a feeder, one or more distribution transformers may be provided to further step down the distribution voltage for a commercial or residential customer. The distribution voltage range may be from 5 through 46 kV, for example. Various connectors are used throughout the distribution system. In particular, the primary side of a distribution transformer typically includes a transformer bushing to which a bushing insert is connected. In turn, an elbow connector may be removably coupled to the bushing insert. The distribution feeder is also fixed to the other end of the elbow connector. Of course, other types of connectors are also used in a typical electrical power distribution system. For example, the connectors may be considered as including other types of removable connectors, as well as fixed splices and terminations. Large commercial users may also have a need for such high voltage connectors.
One particular difficulty with conventional elbow connectors, for example, is that they use curable materials. For example, such a connector may typically be manufactured by molding the inner semiconductive layer first, then the outer semiconductive jacket (or vise-versa). These two components are placed in a final insulation press and then insulation layer is injected between these two semiconductive layers. Accordingly, the manufacturing time is relatively long, as the materials need to be allowed to cure during manufacturing. In addition, the conventional EPDM materials used for such elbow connectors and their associated bushing inserts, may have other shortcomings as well.
One typically desired feature of an elbow connector is the ability to readily determine if the circuit in which the connector is coupled is energized. Accordingly, voltage test points have been provided on such connectors. For example, U.S. Pat. No. 3,390,331 to Brown et al. discloses an elbow connector including an electrically conductive electrode embedded in the insulator in spaced relation from the interior conductor. The test point will rise to a voltage if the connector is energized. U.S. Pat. No. 3,736,505 to Sankey; U.S. Pat. No. 3,576,493 to Tachick et al.; U.S. Pat. No. 4,904,932 to Schweitzer, Jr.; and U.S. Pat. No. 4,946,393 to Borgstrom et al. disclose similar test points for an elbow connector. Such voltage test points may be somewhat difficult to fabricate, and upon contamination and repeated use, they may become less accurate and less reliable.
An elbow connector typically includes a connector body having a passageway with a bend therein. A semiconductive EPDM material defines an inner layer at the bend in the passageway. An insulative Ethylene Propylene Diene Monomer (EPDM) second layer surrounds the first layer, and a third semiconductive EPDM layer or outer shield surrounds the second insulative layer. A first end of the passageway is enlarged and carries an electrode or probe that is matingly received in the bushing insert. A second end of the passageway receives the end of the electrical conductor. The second connector end desirably seals tightly against the electrical conductor or feeder end. Accordingly, another potential shortcoming of such an elbow connector is the difficulty in manually pushing the electrical conductor into the second end of the connector body.
In an attempt to address the difficulty of inserting the electrical connector into the second connector end, U.S. Pat. No. 4,629,277 to Boettcher et al. discloses an elbow connector including a heat shrinkable tubing integral with an end for receiving an electrical conductor. Accordingly, the conductor end can be easily inserted into the expanded tube, and the tube heated to shrink and seal tightly against the conductor. U.S. Pat. No. 4,758,171 to Hey applies a heat shrink tube to the cable end prior to push-fitting the cable end into the body of the elbow connector.
U.S. Pat. No. 5,230,640 to Tardif discloses an elbow connector including a cold shrink core positioned in the end of an elbow connector comprising EPDM to permit the cable to be installed and thereafter sealed to the connector body when the core is removed. However, this connector may suffer from the noted drawbacks in terms of manufacturing speed and cost. U.S. Pat. No. 5,486,388 to Portas et al.; U.S. Pat. No. 5,492,740 to Vallauri et al.; U.S. Pat. No. 5,801,332 to Berger et al.; and U.S. Pat. No. 5,844,170 to Chor et al. each discloses a similar cold shrink tube for a tubular electrical splice.
Another issue that may arise for an elbow connector is electrical stress that may damage the first or semiconductive layer. A number of patents disclose selecting geometries and/or material properties for an electrical connector to reduce electrical stress, such as U.S. Pat. No. 3,992,567 to Malia; U.S. Pat. No. 4,053,702 to Erikson et al.; U.S. Pat. No. 4,383,131 to Clabburn U.S. Pat. No. 4,738,318 to Boettcher et al.; U.S. Pat. No. 4,847,450 to Rupprecht, deceased; U.S. Pat. No. 5,804,630 and U.S. Pat. No. 6,015,629 to Heyer et al.; U.S. Pat. No. 6,124,549 to Kemp et al.; and U.S. Pat. No. 6,340,794 to Wandmacher et al.
For a typical 200 Amp elbow connector, the elbow cuff or outer first end is designed to go over the shoulder of the mating bushing insert and is used for containment of the arc and/or gasses produced during a load-make or load-break operation. During the past few years, the industry has identified the cause of a flashover problem which has been reoccurring at 25 kV and 35 kV. The industry has found that a partial vacuum occurs at certain temperatures and circuit conditions. This partial vacuum decreases the dielectric strength of air and the interfaces flashover when the elbow is removed from the bushing insert. Various manufacturers have attempted to address this problem by venting the elbow cuff interface area, and at least one other manufacturer has insulated all of the conductive members inside the interfaces.
U.S. Pat. No. 6,213,799 and its continuation Application Ser. No. 2002/00055290 A1 to Jazowski et al., for example, discloses an anti-flashover ring carried by the bushing insert for a removable elbow connector. The ring includes a series of passageways thereon to prevent the partial vacuum from forming during removal of the elbow connector that could otherwise cause flashover. U.S. Pat. No. 5,957,712 to Stepniak and U.S. Pat. No. 6,168,447 to Stepniak et al. also each discloses a modification to the bushing insert to include passageways to reduce flashover. Another approach to address flashover is disclosed in U.S. Pat. No. 5,846,093 to Muench, Jr. et al. that provides a rigid member in the elbow connector so that it does not stretch upon removal from the bushing insert thereby creating a partial vacuum. U.S. Pat. No. 5,857,862 to Muench, Jr. et al. discloses an elbow connector including an insert that contains an additional volume of air to address the partial vacuum creation and resulting flashover.
Yet another potential shortcoming of a conventional elbow connector, for example, is being able to visually determine whether the connector is properly seated onto the bushing insert. U.S. Pat. No. 6,213,799 and its continuation Application No. 2002/00055290 A1 to Jazowski et al., mentioned above, each discloses that the anti-flashover ring on the bushing insert is colored and serves as a visual indicator that the elbow connector is seated when the ring is obscured.
U.S. Pat. No. 5,641,306 to Stepniak discloses a separable load-break elbow connector with a series of colored bands that are obscured when received within a mating connector part to indicate proper installation. Along these lines, but relating to the electrical bushing insert, U.S. Pat. No. 5,795,180 to Siebens discloses a separable load break connector and mating electrical bushing wherein the busing includes a colored band that is obscured when the elbow connector is mated to a bushing that surrounds the removable connector.
Accordingly, there exists several significant shortcomings in conventional electrical connectors, particularly for high voltage distribution applications.
In view of the foregoing background, it is therefore an object of the invention to provide an electrical connector and associated manufacturing method, particularly for high voltage applications, with a reliable voltage monitor point.
This and other objects, features and advantages in accordance with the invention are provided by an electrical connector comprising a connector body having a passageway therethrough and including a first layer adjacent the passageway, a second layer surrounding the first layer, and a third layer surrounding the second layer. The third layer may be arranged in three spaced apart portions with first and third portions to be connected to a reference voltage so that the second portion floats at a monitor voltage for the electrical connector. The first and third layers may have a relatively low resistivity, and the second layer may be an insulator having a relatively high resistivity. At least one of the layers preferably comprises a thermoplastic elastomer (TPE) material. In particular, the second layer may comprise an insulative TPE material, and the third layer may comprise a semiconductive TPE material. In some embodiments, the first layer may also comprise a semiconductive TPE material. The TPE material layers may be overmolded to thereby increase production speed and efficiency thereby lowering production costs. The TPE material may also provide excellent electrical performance and permit the ready manufacturing of the voltage monitor area for the connector as provided by the split shield configuration.
The electrical connector may further comprise a monitor point extending outwardly from the second portion of the third layer. A cover may also be provided over the second portion of the third layer that permits access to the monitor point. The second portion of the third layer may have a band shape, for example. Reliable and accurate voltage sensing is thus provided.
The passageway may have first and second ends and a medial portion extending therebetween. The first layer may be positioned along the medial portion of the passageway and spaced inwardly from respective ends of the passageway. For elbows and T-connectors, the medial portion of the passageway may have a bend therein. The first end of the passageway may also have an enlarged diameter to receive an electrical bushing insert therein for some embodiments.
For other embodiments, the connector body may have a tubular shape defining the passageway. The first layer may have at least one predetermined property to reduce electrical stress. For example, the predetermined property may comprise a predetermined impedance profile. Alternately or additionally, the predetermined property may comprise a predetermined geometric configuration, such as one or more ribs extending outwardly adjacent the bend in those embodiments including the bend.
The first layer may define an innermost layer, and the third layer may define an outermost layer. The connector may also include at least one pulling eye carried by the connector body. The connector body may be configured for at least 15 KV and 200 Amp operation. Each of the first and third layers may have a resistivity less than about 108 xcexa9xc2x7cm, and the second layer may have a resistivity greater than about 108 xcexa9xc2x7cm.
A method aspect of the invention is for making an electrical connector body having a passageway therethrough. The method may comprise providing a first layer to define at least a medial portion of the passageway; overmolding a second layer surrounding the first layer and comprising an insulative TPE material having a relatively high resistivity; and overmolding a third layer surrounding the second layer and comprising a material having a relatively low resistivity. The third layer may be arranged in three spaced apart portions with first and third portions to be connected to a reference voltage so that the second portion floats at monitor voltage for the electrical connector. A monitor point may be formed onto the third layer and a cover also formed thereover, yet permitting access to the monitor point. The third layer may also comprise a semiconductive TPE material, and the first layer may comprise a semiconductive TPE material in some embodiments.